Although the costs of petroleum feedstocks have remained relatively stable in recent years, it is only a matter of time when the world will again be faced with shortages and escalating prices. Consequently, there is still a substantial amount of research being conducted on processes for producing synthetic fuels, including catalytic coal conversion, or liquefaction. Both supported and non-supported catalysts have been used. It is known in the art that supported hydroconversion catalysts are not as active for the conversion of coal to liquid products as are high surface area, well dispersed, non-supported slurry catalysts. Particularly active catalyst systems in the art, for the liquefaction of coal, are well dispersed transition metal, especially molybdenum, containing slurry catalysts. These catalysts are typically prepared by dispersing a Mo-containing catalyst precursor in a coal/solvent slurry, then thermally forming and activating the catalyst in-situ by the introduction of sulfur, either as solid sulfur, or as a sulfur-containing gas, typically H.sub.2 S. See, for example, U.S. Pat. No. 4,369,106.
It is taught in U.S. Pat. No. 4,077,867 that coal, in a hydrogen donor diluent, is liquefied in the presence of a catalyst prepared in situ in the coal/hydrogen-donor mixture from catalyst precursors which may be heteropolyacids, such as phosphomolybdic acid. Further, U.S. Pat. No. 4,136,013 teaches a coal liquefaction process wherein an emulsion of an aqueous solution of a metal salt in a water-immiscible liquid medium is added to the coal slurry. The metal salt is a water soluble salt such as or alkali metal heptamolybdate. However, owing to the difficulty of recovering the expensive transition metal after use, inexpensive metals, such as iron, have continued to be of interest as the basis of an economical and disposable catalyst for coal liquefaction.
Various studies have shown that the catalytic activity, obtained by conventional methods for the direct introduction of iron into the feed slurry, in the form of readily available minerals, such as oxide, or sulfide concentrates, is generally low. Such approaches are typically ineffective, or only moderately effective even when relatively high concentrations (wt. % based on coal) of iron are used. Among the factors contributing to such low activity are poor initial dispersion and relatively low surface area of the added iron phase. Another contributing factor is a tendency for the iron minerals to aggregate at conversion conditions, resulting in a loss of surface area of the catalytically active iron species.
A variety of techniques have been used to increase the initial dispersion of iron in coal. For example, catalyst precursors have been introduced by physically mixing very small particles of iron into the feed slurry. Iron has also been deposited by thermal decomposition of oil soluble forms. Iron has also been impregnated into the coal by use of water soluble iron compounds. When solubility is the means of contact with coal, catalyst precursors must of necessity possess relatively high solubility, e.g., in excess of several weight percents in water or oil. Two shortcomings with these approaches are that the iron ends up outside of the coal, and the resulting dispersion is not optimal, that is, it is not atomic.
One effective, but non-economical, method for dispersing iron in coal is taught in Effect Of Activation Conditions On Dispersed Iron Catalysts in Coal Liquefaction; by A. V. Cugini et al; U.S. Department of Energy; Preprint Paper--Americal Chemical Society, Division Fuels Chemistry, 36(1); pp 91-102, 1991. The process of that paper comprises combining both coal impregnation and precipitation techniques to generate a highly active dispersed iron catalyst having considerable promise for liquefaction. While such aforementioned techniques have met with varying degrees of success, there still remains a need in the art for even more effective and economical methods of dispersing catalytic metals into coal particles. None of the prior art methods can achieve the dispersion of the present invention wherein atomic dispersion of metal is accomplished by contacting the carbonaceous material with a metal precursor in a single step.